Currently used techniques for color projection displays tend to be relatively inefficient in their light utilization. Such low efficiency limits the brightness of the display, which in effect limits the acceptable amount of ambient lighting in a viewing environment.
In certain presently used designs, light from a spectrally broad source is collected by a condensing lens and illuminates a spatial light modulator system. The spatial light modulator system comprises a two-dimensional array of pixels and the amount of light transmitted through each pixel is controlled electronically. A projection lens then images the array of pixels on a viewing screen, the magnification of the displayed image being determined by the particular characteristics of the projection lens. The light impinging on each pixel of the spatial light modulator is spectrally broad (i.e., white light). Therefore, unless the system is modified to distinguish colors, the display is only capable of displaying black and white images.
In many current systems used to modify such a system so that it is capable of displaying color images, each pixel of the spatial light modulator is divided into three sub-pixels having equal areas. Each of the three sub-pixels is covered with a micro-color filter having a different spectral transmittance. For example, the filters are chosen such that one filter transmits only red light, another filter only green light, and the third filter only blue light. The transmittances of the three sub-pixels of each pixel of the spatial light modulator can be controlled independently, resulting in the ability to display a color image.
The inefficiency of the approach can be seen by considering the following factors. The light illuminating a full pixel essentially is white light and, consequently, the light impinging each sub-pixel is also white light. The red filtered sub-pixel transmits only red light, absorbing all of the incident green and blue light. Likewise, the other two sub-pixels transmit only its corresponding color, absorbing the other two colors. It is apparent that this approach utilizes, at most, only one third of the available light impinging on the modulator, and absorbs the rest.
Furthermore, state-of-the-art microcolor filters required to produce acceptable color images are approximately only 33% efficient in transmitting the color that they are designed to transmit. Therefore the overall light utilization of current color projection displays is about 10%.
One approach for improving the efficiency of color projection displays is found in U.S. Pat. No. 5,161,042 issued on Nov. 3, 1994 to H. Hamoda. In accordance therewith, the spectrally broad input light is supplied to three dichroic mirrors which reflect three different color components, e.g., red, green, and blue, in different directions, i.e., at different angles with respect to each other. The reflected components are then supplied to an array of lenses for focusing the different color components so as to converge light beams of similar wavelength ranges for transmission through a liquid crystal display element so as to form combined color images on a display screen. A further U.S. Pat. No. 5,264,880, issued on Nov. 23, 1993, to R. A. Sprague et al., discloses a similar approach to that of Hamoda wherein the dichroic mirrors are replaced by a phase grating for dispersing the color components of light received thereat into a spectrum of different colors at different angles relative to each other.
While such approaches can be used, the losses of energy of each color component are sufficient to reduce the efficiencies of such systems and to show the need for further improvement in such display systems. Such improved display systems should minimize such losses so as to provide for substantially the total use of the received energy across the color spectrum in the imaging display process resulting in an improvement of the efficiency of the system.
The invention relates to a color projection display in which received light, having a relatively broad spectrum, illuminates a multi-level optical phase grating so as to disperse each of the color components contained therein into a plurality of different diffraction orders. In one embodiment, the diffraction orders of each color component are then focussed onto a zero-order phase shift element which phase shifts only the undiffracted light (i.e., the zero diffraction order) with respect to the diffracted light (i.e., the higher level diffraction orders). The output of the zero-order phase shifter is then imaged onto a display having a plurality of pixels, each pixel having sub-pixel regions assigned to transmit different color components of light. The depths of the phase grating element and the zero-order phase shifter are suitably selected so they are practical for manufacture and so the area of chromaticity space for the color components at the image plane is maximized.
The use of such a combination of multi-level phase grating and a zero-order phase shifter, having suitably determined depths, provides desired color components at each pixel in which essentially little or no energy is lost. These color components are then suitably combined to provide a color image at each of the pixels of the display which is considerably brighter than that available using prior known systems.
In another embodiment, a broad spectrum light source illuminates a multilevel optical phase element which disperses the broad spectrum light from the light source by diffraction. A display having a number of pixel elements, each capable of transmitting a predetermined spectral region, is positioned within the near field region of the multilevel optical phase element so as to receive the light dispersed by the multilevel phase element. In one embodiment, the multilevel phase element is periodic in two dimensions, thereby concentrating the light in two dimensions.
In yet another embodiment, a method for displaying a color image is disclosed. The method for displaying a color image includes illuminating a multilevel optical phase element with a broad spectrum light source. The multilevel phase element disperses light from the light source by diffraction. A display having a plurality of pixel elements, each transmitting a predetermined spectral region, is positioned within the near field region of the multilevel optical phase element to receive the dispersed light from the multilevel optical phase element.
Preferred embodiments of the invention include transmissive active matrix liquid crystal display devices. An active matrix array of transistor circuits and pixel electrodes is bonded to an optically transmissive substrate by a layer of adhesive material. A layer of liquid crystal material is disposed over the active matrix array to form a transmissive display structure such that actuation of the pixel electrodes controls transmission of light through a respective volume of the liquid crystal material. A diffractive optical element is also aligned with the active matrix array to disperse light of different colors through different volumes of the liquid crystal material. The active matrix array and the diffractive optical element are mounted within a display or projector housing to form a direct-view or projected-view display device.
Preferred embodiments of the invention also include an array of light reflective pixel electrodes such as digital micromirror display devices. In a digital micromirror display device, a matrix housing contains a digital micromirror array of light reflective, electromechanical pixels and a diffractive optical element. The diffractive optical element is aligned with the digital micromirror array to disperse light of different colors onto different electromechanical pixels. Instead of electromechanical pixel electrodes, a reflector liquid crystal display device can be used.